1 Bui Xuan Thanh et al. / Journal of Water Sustainability 2 (2011) 13-23

* Corresponding to: bxthanh@hcmut.edu.vn

Removal of Non-Biodegradable Organic Matters from Membrane

Bioreactor Permeate By Oxidation Processes

Bui Xuan Thanh*, Vo Thi Kim Quyen, Nguyen Phuoc Dan

Faculty of Environment, Ho Chi Minh City University of Technology, Vietnam

ABSTRACT

This study aimed to optimize the operational parameters of oxidation processes in removing the non-biodegradable

matters from the permeate of membrane bioreactor (MBR) which is used to treat dyeing and textile wastewater. The

MBR was operated at the OLR of 1.3–1.7 kg COD/m3.day, HRT of 11 hours, and SRT of 60 days. The quality of

MBR permeate were stable during operation period with COD and colour of 107±7 mg/L (96-114 mg/L) and 85±10

Pt-Co (70-90 Pt-Co), respectively. The insight of the advanced oxidation processes (AOPs) is to generate hydroxyl

free radical (OH·) as strong oxidant to decompose the recalcitrants which cannot be oxidized by the conventional

biological processes. The results show that the single ozone oxidation process removed 50% of COD and 41% of

colour with the ozone generation rate of 104 mg O3/h for 25 minutes at pH 9. For the Peroxone oxidation, about

59% of COD and 53% of colour removal were achieved at the same operating conditions at pH 8.5, and molar ratio

H2O2/O3 of 1:2. While for the O3/UV oxidation, COD and colour reduced to 55% and 54% after 25 minutes at pH 9.

The Fenton oxidation achieved removal efficiency of 90% of colour and 84% of COD at pH 3 and the mass ratio

Fe2+

/H2O2/COD of 1:1:1. It indicates that the three oxidation processes could removed refractory residue from

MBR permeate to meet the Vietnam National Technical Regulation for dyeing and textile wastewater (level B,

QCVN 13:2008/BTNMT). In general, the single ozonation shows advantages in term of simple operation, reasona-

ble treatment cost and rational removal color and part of non-biodegradable COD among the other AOPs. During

ozonation process, non-biodegradable COD in MBR permeate converted to biodegradable COD fraction which can

be further mineralized by biological process.

Keywords: Advanced oxidation processes (AOPs); Hydroxyl free radical; Ozone; Peroxone; O3/UV; Fenton;

Dyeing and textile wastewater

1. INTRODUCTION

Dyeing and textile industry needs a lot of

water for production processes and generates

large quantity of wastewater from various

steps of dyeing processes, about 12-300

m3/ton of fabric. Main pollution in dyeing and

textile wastewater came from dyeing and

finishing processes. The wastewater contains

high amount of dyestuffs, which generally are

organic compounds with complex structure,

salts, heat, enzymes, surfactants, oxidizing

and reducing agents. This wastewater has

high COD deriving from additives, suspended

solid (SS). In addition, only 47% of 87 of

dyestuff are biodegradable. It is documented

that residual colour is usually due to insoluble

dyes which have low biodegradation as

reactive blue 21, direct blue 80 and vat violet

(Adel et al., 2005). Because of those, dyeing

Journal of Water Sustainability, Volume 1, Issue 3, December 2011, 289–299

© University of Technology Sydney & Xi’an University of Architecture and Technology

290 B. X. Thanh et al. / Journal of Water Sustainability 3 (2011) 289-299

and textile wastewater is an important pollu-

tion source to be concerned.

COD values of dyeing and textile

wastewater are high compared to other pa-

rameters. The concentration of COD varies

from 150–12,000 mg/L and colour fluctuates

between 50–2,500 Pt-Co. In most cases, the

BOD5/ COD ratio is around 0.25 that implies

that the wastewater contains large amount of

non-biodegradable organic matter (Adel et al.,

2005). Many treatment processes were made

for treating dyeing and textile wastewater

such as chemical oxidation, liquid-liquid

extraction, adsorption, ultra-filtration, reverse

osmosis, and biological treatment. However,

because of the complex structure of organic

compounds and the low ratio of BOD5/COD

which translates to a hardly biodegradable,

the conventional treatment methods have

found difficulty to mineralize dyestuffs and

complex structure of organic compounds

(Adel et al., 2005; Konsowa, 2003). To ease

the stated problems, advanced oxidation

processes (AOPs) have been developed.

AOPs defined by Glaze et al. (1987) are

involving the use of ozone (O3), hydrogen

peroxide (H2O2), UV radiation and Fenton

agent to generate and use hydroxyl free

radical (OH•) as a strong oxidant to destroy

compound that cannot be oxidized by conven-

tional oxidant (Adel et al., 2005). Hydroxyl

radicals are extraordinarily reactive species

that attack most of the organic molecules. Its

oxidizing potential is quite strong more than

chlorine 2.05 times (Carey, 1992).

The importance of these processes is due to

the high reactivity and redox potential of this

free radical that reacts non-selectively with

organic compounds present in water. These

processes present a high degree of flexibility

because they can be used individually or in

combination depending on the problem to be

solved. Generation of OH• is commonly

accelerated by combining O3, H2O2, TiO2, UV

radiation, electron-beam irradiation and

ultrasound. Of these, O3/H2O2 and O3/UV

hold the greatest promise to oxidize dyeing

and textile wastewater. Another advantage of

the AOPs is that they may be applied under

mild conditions (atmospheric ambient pres-

sure and room temperature).

1.1 Ozone Oxidation Process

Ozone application can be generalized into two;

a powerful disinfection and a strong oxidant

agent for water and wastewater to remove

colour, odour, eliminating trace toxic synthet-

ic organic compounds. Once dissolved in

water, ozone reacts with a great number of

organic compounds in two different ways: by

direct oxidation as molecular ozone or by

indirect reaction through formation of sec-

ondary oxidants like hydroxyl radical OH•

(Baig and Liechti, 2001).

3O3 + OH-

+ H+ → 2OH

• + 4O2 (1)

Colour removal using ozonation from dye-

ing and textile wastewater depends on dye

concentration (Sheng and Chi, 1993; Mehmet

and Hasan, 2002; Konsowa, 2003). Higher

initial dye concentration of dyeing and textile

wastewater causes more ozone consumption.

Increasing ozone concentration enhances

mass transfer that causes an increase in ozone

concentration in liquid phase, which increase

colour removal. The other possible explana-

tion is that more intermediates, which con-

sume more ozone, are generated when the

initial dye concentration is high. Gianluca and

Nicola (2001) reported that colour removal of

biotreated dyeing and textile wastewater

depended on initial COD of the wastewater.

In addition, colour removal efficiency in-

creased with increasing the temperature from

25oC to 50

oC. Alkaline pH was also found as

favorable condition for high removal of

colour and COD.

Ozone is the basic compound for many

oxidation processes included under the

general term of ozonation. In these processes,

B. X. Thanh et al. / Journal of Water Sustainability 3 (2011) 289-299 291

ozone may be used alone or with other agents

such as hydrogen peroxide, UV radiation,

catalysts, ultrasound, activated carbon, etc.

1.2 O3/UV Oxidation Process

According to Rein (2001), conventional

ozonation of organic compounds does not

completely oxidize organics to CO2 and H2O

in many cases. Hung-Yee and Ching-Rong

(1995) documented O3/UV as the most

effective method for decolorizing of dyes

comparing with oxidation by UV or ozonation

alone. The O3/UV system is also an effective

method for the destruction of organic com-

pounds in water. It makes use of UV photons

to activate ozone molecules, thereby facilitat-

ing the formation of hydroxyl radicals. UV

lamp must have a maximum radiation output

254 nm for an efficient ozone photolysis. The

photodecomposition of ozone leads to two

hydroxyl radicals.

O3 + hν + H2O → H2O2 + O2 (2)

H2O2 + hν → 2OH• (3)

Even though ozone can be photodecom-

posed into hydroxyl radicals to improve the

degradation of organics, UV light is highly

absorbed by dyes and very limited amount of

free radical (OH•) can be produced to decom-

pose dyes. In normal cases, ozone itself will

absorb UV light, competing with organic

compounds for UV energy. However, O3/UV

treatment is recorded to be more effective

compared to ozone alone, in terms of COD

and colour removals. Azbar et al. (2004)

stated that using O3/UV process high COD

and colour removals would be achieved under

basic conditions.

1.3 O3/H2O2 Oxidation Process

The addition of both hydrogen peroxide and

ozone to wastewater accelerates the decompo-

sition of ozone and enhances production of the

hydroxyl radical. Hydrogen peroxide in

aqueous solution is partially dissociated in the

hydroperoxide anion (HO2-), which reacts with

ozone, decomposing this and giving rise to a

series of chain reactions with the participation

of hydroxyl radicals. In the global reaction

two ozone molecules produce two hydroxyl

radicals (Glaze and Kang, 1989).

H2O2 + 2O3 → 2OH• + 3O2 (4)

Arslan et al. (2002) documented that O3/

H2O2 treatment of synthetic dye-house

wastewater highly depended on the pH of the

effluent. At acidic pH, H2O2 reacts only very

slowly with O3 whereas at pH values above 5

a strong acceleration of O3 decomposition by

H2O2 has been observed. At higher pH, even

very small concentration of H2O2 will be

dissociated into HO2¯ ions that can initiate the

ozone decomposition more effectively than

OH- ion (Staehlin and Hoigne, 1982; Glaze

and Kang, 1989). As a result, the ozone

decomposition rate will increase with increas-

ing pH. However, hydrogen peroxide in

alkaline medium reacts with sodium hydrox-

ide. As a result, lower concentrations of

hydrogen peroxide are available for the

formation of hydroxyl radicals. The inhibitory

performance of O3/H2O2 process on microbial

growth depended on the H2O2 to O3 mass

ratio. This ratio ranged from 0.3 to 0.6 for

different type of dyes (Rein, 2001).

1.4 Fenton Oxidation Process

The term of Fenton reagent refers to aqueous

mixtures of Fe(II) and hydrogen peroxide

(H2O2). The Fenton reaction was discovered

by Fenton (1894). Forty years later, the

Haber-Weiss (1934) mechanism was postulat-

ed, which revealed that the effective oxidative

agent in the Fenton reaction was the hydroxyl

radical (OH·). Although the Fenton reagent

has been known for more than a century and

shown to be a powerful oxidant, the mecha-

nism of the Fenton reaction is still under

intense and controversial discussion. Genera-

292 B. X. Thanh et al. / Journal of Water Sustainability 3 (2011) 289-299

tion of OH· radicals by the reaction of H2O2

with ferrous salt has been the subject of

numerous studies during the last decade

(Arnold et al., 1995). Additional important

reactions occurring in aqueous mixtures of

iron and hydrogen peroxide under acidic

conditions include the following:

Fe2+

+ H2O2 Fe+ + OH

• + OH

– (5)

Fe3+

+ H2O2 Fe2+

+ HO2• + H

+ (6)

OH• + Fe

2+ OH

– + Fe

3+ (7)

OH· + H2O2 H2O + HO2• (8)

Fe2+

+ HO2 • Fe

3+ + HO2

- (9)

Fe3+

+ HO2• Fe

2+ + O2 + H

+ 10)

Several studies have demonstrated that the

best oxidation efficiency is achieved when

neither H2O2 nor Fe2+

is overdosed, so that the

maximum amount of OH• radicals are availa-

ble for the oxidation of organics. Many

authors suggested Fe2+

to H2O2 mass ratio to

be optimal at 1 to 10, but it must be optimized

for particular wastewater.

2. MATERIALS AND METHODS

Raw wastewater was collected from a dyeing

factory in Ho Chi Minh City. The wastewater

was treated biologically from a submerged

membrane bioreactor (MBR) which was

operated at OLR of 1.3–1.7 kg COD/m3.day,

HRT of 11 hours and SRT of 60 days. The

membrane permeate which contains the COD

of 107±7 mg/L (96-114 mg/L) and 85±10 Pt-

Co (70-90 Pt-Co) was used for different

oxidation processes.

For Ozone oxidation, 1.6 L of wastewater

was placed in the reactor (Figure 1 without

UV lamps) and then ozone was supplied by

ozone generator with capacity of 104 mg O3/h.

The duration of ozone supplied was varied

from 5, 10, 15, 20, 25, 30, 35 and 40 minutes.

The pH values changed from 7 to 10 to study

the effect of this factor on the degradation

efficiency. At the end of the reaction time,

each sample was taken and analyzed for the

concentrations of COD and colour in order to

assure the mineralization of the wastewater.

The studies for O3/UV and O3/H2O2 oxida-

tions were similar to ozone oxidation. For

O3/UV oxidation, two UV lamps (6 Watt)

were added inside the reactor (Figure 1). And

for O3/H2O2 oxidation, different volumes of

H2O2 30% solution were investigated to

determine the removal efficiency. The molar

ratio of O3/H2O2 was varied from 0.3; 0.4; 0.5;

0.6; 0.7 and 0.8.

Figure 1 Experimental setup of advanced oxidation processes (O3, O3/UV and O3/H2O2)

B. X. Thanh et al. / Journal of Water Sustainability 3 (2011) 289-299 293

For Fenton oxidation, a 400 mL sample

was placed into a 500 mL beaker, pH was

adjusted prior to chemical oxidation experi-

ments. The pH values were changed from 2 to

5. The FeSO4.7H2O was added to attain

selected mass ratio Fe2+

: H2O2 (1:5, 1:4, 1:3,

1:2, 1:1 and 2:1). Finally, Fenton reaction was

started with addition of H2O2 (30%) to

achieve the mass ratio (1:5, 1:4, 1:3, 1:2, 1:1

and 2:1). The aqueous solution of Fenton

reagent and wastewater were stirred during

the reaction period with the stirring speed of

60 rpm. At the end of the reaction time, each

sample was taken and analyzed for the con-

centrations of COD and colour in order to

determine the removal efficiencies of pollu-

tants.

The COD fractions including biodegradable

and slowly biodegradable were measured by a

batch respirometer with the So/Xo ratios

ranging from 0.01-0.03 mg COD/mg VSS.

The sludge of 1500 mgVSS/L was taken from

conventional activated sludge process. The

biodegradable COD and slowly biodegradable

COD were quantified based on the oxygen

consumption rate during the experiment. The

non-biodegradable COD was estimated by the

subtraction from the initial COD. This COD

fraction was according to Mathieu and

Etienne (2000). This experiment was triplicate

for each wastewater sample before and after

ozonation.

3. RESULTS AND DISCUSSION

3.1 Ozone Oxidation

Figure 2 shows that the COD and colour

removal using ozonation are dependent on pH

values. The rate of oxidation increased slight-

ly with increasing solution pH. At pH 9, 50%

of COD and 41% of colour were removed for

25 minutes and ozone generating rate of 104

mg O3/h. The results show that 0.4–0.5 g

ozone would treat 1 g COD, and the efficien-

cy of ozone usage was 70%.

Figure 2 Removal efficiency at different pH by ozone oxidation

294 B. X. Thanh et al. / Journal of Water Sustainability 3 (2011) 289-299

Higher COD and colour removal at alkaline

pH could be due to enhancement of ozone

decomposition by hydroxyl radical. The rate

of ozone decomposition is favoured by the

formation of hydroxyl radicals at higher pH

values. Arslan et al. (2002) reported that high

colour removal of simulated reactive dye bath

effluent was achieved at pH 7 when using

ozone concentration of 2,970 mg/L and

remained unchanged at pH 11.

The removal of COD depends on the

strength of dye waste, where COD reduction

was low with the medium- and high-strength

dye waste. The low COD reduction is at-

tributable to the fact that the structured

polymer dye molecules (non-biodegradable

COD) are broken by ozonation to small

molecules, such as acetic acid, aldehyde,

ketones, etc. instead of CO2 and water (Sheng

and Chi, 1993).

3.2 O3/UV Oxidation

The trend of COD removal is similar with that

of ozone oxidation process (Figure 3). At pH

9, 55% of COD and 54% of colour were

removed for 25 minutes and ozone generating

rate of 104 mg O3/h. The results show that

0.3–0.4 g ozone would treat 1 g COD, and the

efficiency of ozone usage was 58%.

The O3/UV process makes use of UV pho-

tons to activate ozone molecules, thereby

facilitating the formation of hydroxyl radicals.

So the O3/UV oxidation is more effective

compared to ozone alone. For 25 minutes of

reaction, the O3/UV treatment could remove

55% of COD while the single ozone oxidation

reduced only 50%. Bes-Piá et al. (2003)

reported that O3/UV treatment of biologically

treated dyeing and textile wastewater reduced

COD from 400 to 50 mg/L for 30 minutes.

While using ozone oxidation alone, COD

reduced to 286 mg/L in same duration for the

same operating conditions. Azbar et al. (2004)

documented that using O3/UV process high

COD removal would be achieved under basic

conditions (pH 9).

Figure 3 Removal efficiency at different pH by O3/UV oxidation

B. X. Thanh et al. / Journal of Water Sustainability 3 (2011) 289-299 295

3.3 O3/H2O2 Oxidation

Similar to ozone oxidation, at alkaline pH of

7-8.5, the efficiency of COD and colour

removal is higher (Figure 4). At pH 8.5 about

59% of COD and 53% of colour removal

were achieved for 25 minutes and ozone

generating rate of 104 mg O3/h. The results

indicate that 0.24–0.40 g ozone would treat 1

g COD, and the efficiency of ozone usage was

59%.

The decolorization efficiency increases

with increasing hydrogen peroxide dose up to

a point where it reaches a maximum and then

starts to decrease. This is due to the fact that

hydrogen peroxide is a scavenger for hydrox-

yl radicals according to the reaction given in

the following equation (Dionysiou et al.,

2004):

OH• + H2O2 → HO2

• + H2O (11)

2HO2• → H2O2 + O2

• (12)

OH• + OH

• → H2O2 (13)

When enough hydrogen peroxide is present

in the solution, it starts to compete with the

dye for reaction with hydroxyl radicals since

HO2• is less reactive than the OH

• radical. An

increased level of hydrogen peroxide has a

diminishing effect on the reaction rate (Ale-

boyeh et al., 2003). Therefore, it is important

to optimize the applied dose of hydrogen

peroxide. The molar ratio H2O2/O3 of 1:2 was

determined by many experiments in this study.

Many researchers (Glaze, 1989; Singer and

Reckhow, 1999 and Rein, 2001) stated that

the molar ratio H2O2/O3 ranged from 0.3-0.6

for different types of dyes.

3.4 Fenton Oxidation

In this study, Fenton treatment could achieve

removal efficiency of 90% of colour and 84%

of COD at pH 3 with the mass ratio

COD/H2O2/Fe2+

1:1:1. The reaction of Fe (II)

and hydrogen peroxide occurs under acidic

condition. This result shows the removal of

COD and colour was highest at pH 3 which is

similar to other researchers.

Figure 4 Removal efficiency at different pH by O3/H2O2 oxidation

296 B. X. Thanh et al. / Journal of Water Sustainability 3 (2011) 289-299

Figure 5 shows the removal efficiency of

COD and colour by Fenton oxidation at pH 3,

mass ratio H2O2/COD 1:1 and values of mass

ratio Fe2+

/H2O2. The maximum amount of

OH• radicals is available for the oxidation of

organics, neither H2O2 nor Fe2+

is overdosed.

This study suggested Fe2+

to H2O2 mass ratio

to be optimal at 1:1. When dosage of hydro-

gen peroxide and Fe (II) are too high, they

starts to compete with the dye and the organic

carbon for reaction with hydroxyl radicals

according to equations (7) and (8).

3.5 Removal efficiency of COD and color

of the four oxidation processes

Figure 6 shows that Fenton oxidation was the

most effective process in removing COD and

colour from MBR treating dyeing and textile

wastewater. The removal of COD and color

was almost similar for the ozone combining

processes. The Fenton process is the most

effective but it is costly and complicated due

to chemical usage, sludge production and tank

construction. The costs of wastewater treat-

ment for were 0.20, 0.22, 0.35 and 0.24

USD/m3 of MBR permeate for the O3,

O3/H2O2, O3/UV and Fenton respectively.

Because the removal efficiencies of other

ozone based processes were almost similar,

the single ozone oxidation was selected as the

appropriate process in coupling with MBR

because the following reason: (a) this single

ozone oxidation can treat the MBR permeate

reaching Vietnamese discharge limits with the

lowest cost among the AOPs; (b) there is no

use of chemical or UV equipment in the

processes like perozone, Fenton and O3/UV;

and (c) there is no sludge production and

additional reactors like Fenton process. The

sludge production from Fenton process was

observed approximately 0.1 m3/m

3 of

wastewater (after 30 minute of settling) which

could generate a burden in sludge manage-

ment of the treatment plant. In general, the

single ozone oxidation process is the most

suitable process coupling with MBR treating

dyeing and textile wastewater.

Figure 5 Removal efficiency at different Fe

2+/H2O2 ratios by Fenton oxidation

B. X. Thanh et al. / Journal of Water Sustainability 3 (2011) 289-299 297

Figure 6 Removal efficiency of COD and colour of AOPs

Figure 7 Fate of COD during ozone oxidation

Figure 7 shows the fate of COD in MBR

permeate before and after ozone oxidation. It

reveals that dye macro-molecules were

broken to smaller molecules, thus a part of

non-biodegradable COD transformed into

biodegradable one. This result found that the

oxidation process did not mineralize all

organic matter completely, i.e. into carbon

dioxide and water. For the ozone oxidation,

organic matter was partly mineralized and

partly transformed. The biodegradable COD

was increased from 3% into 24 % after ozone

addition. Thus, the suitable operation for

dyeing and textile wastewater treatment is to

combine the ozone oxidation and biological

process. The ozonized flow could be recycled

to biological reactor for further degradation.

CONCLUSIONS

The AOPs process could remove COD resi-

due and color from the MBR permeate during

25 minutes and ozone generation rate of 104

mg O3/h. The single ozone oxidation process

removed 50% of COD and 41% of colour.

The Peroxone oxidation eliminated about

59% of COD and 53% of colour. The O3/UV

oxidation rejected 55% of COD and 54% of

color. The Fenton oxidation achieved removal

efficiency of 90% of colour and 84% of COD

298 B. X. Thanh et al. / Journal of Water Sustainability 3 (2011) 289-299

at pH 3 and the mass ratio Fe2+

/H2O2/COD of

1:1:1. The single ozone oxidation was found

to be appropriate operation coupling with

MBR in terms of removal efficiency, treat-

ment cost and simple operation in real prac-

tice.

The three oxidation processes could re-

moved refractory residue from MBR perme-

ate to meet the Vietnam National Technical

Regulation for dyeing and textile wastewater

(level B, QCVN 13:2008/BTNMT).

For ozone oxidation process, organic mat-

ters in MBR permeate was partly mineralized

and partly transformed. Non-biodegradable

organic compounds were converted to biode-

gradable fractions. The biodegradable COD

was increased from 3% into 24 % after ozone

addition.

ACKNOWLEDGEMENT

The authors would like to thanks for the

research grant from JICA-SUPREME project

(B2-12). We are grateful to Ms. N.T.M. Hien,

Mr. N.T. Tin, Mr. H.V. Thuan and Mr. H.L.

Cuong for their experimental support.

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